Cloths of this type, so-called wires, are well-known in the art. Many wire types are manufactured, one-layer and multi-layer wires, for instance. Amongst multi-layer wires, two-layer wires are perhaps the best known, and these have been used for a long time in the paper making industry.
Double-layer wires have many advantages over single layer fabrics, because the paper side and the wear side of a fabric can be made to have different properties. Normal constructions of double-layer wires have a smooth paper side and a relatively coarse wear side. The disadvantage of two-layer wires is that they have poor dewatering properties when the warp and weft yarns are woven with high density. This causes problems with paper machines leading to high drag loads.
The principal object of the invention is to reduce paper marking and drag loads caused by the wire. This is achieved by means of a paper machine cloth in which the fabric comprises two longitudinal yarn systems and at least three transverse yarn systems, the first transverse yarn system being formed of yarns of a first diameter and being situated on the paper side of the wire, the second transverse yarn system being formed of yarns of a second diameter, and the third transverse yarn system being formed of yarns of a third diameter, which is substantially smaller than the second diameter, the first longitudinal yarn system being interwoven with the yarns of the first and second transverse yarn systems and the second longitudinal yarn system being interwoven with the yarns of the first and third transverse yarn systems, the second longitudinal yarn system being protected from wearing by placing the third transverse yarn system substantially inside the fabric in order to keep the lower knuckles of the second longitudinal yarn system inside the fabric as well.
The advantage of such a paper machine fabric is that it has a higher permeability and lower drag loads than conventional double-layer wires, but it also has a good wear resistance and a non-marking paper side surface and a stable structure.
Wires with conventional construction have longitudinal yarns which weave with every transverse yarn system both on paper side and the wear side. High yarn density in these prior art fabrics usually leads to drainage problems and low yarn density leads to increased sleaziness. Several attempts have been made to improve drainage, wear resistance and marking properties, for example by means of using more than one longitudinal yarn system or by using transverse yarns of alternating diameter on the wear side of the fabric, but so far improvements in some properties have caused deterioration in other properties. The present invention improves both dewatering properties and stability compared to prior art wires.
One application, U.S. Pat. No. 4,499,927, has two separate longitudinal yarn systems, one of which is interwoven with transverse yarn systems of both the paper side and the wear side, and one only with the transverse yarn systems of the paper side. This application aims to protect some of the longitudinal yarns from abrasion wear but at the same time causes increased sleaziness because of a less stable weave construction compared to conventional two-layer wires.
Another prior art construction with longitudinal yarns interwoven with only one side of the fabric is presented in U.S. Pat. No. 4,314,589, where in one embodiment of the invention some of the longitudinal yarns are interwoven with only the lower layer of transverse yarns, and in another embodiment some of the longitudinal yarns are interwoven with only the upper layer of transverse yarns and some longitudinal yarns are interwoven with only the lower layer of transverse yarns. Another characteristic of this invention is a lower number of wear side transverse yarns compared to the number of paper side transversal yarns. This is said to improve drainage, but together with the longitudinal yarns not interwoven with both layers of transversal yarns it also causes sleaziness.
Transversal yarns of alternating diameters have also been used before. An example of this is the invention of U.S. Pat. No. 3,851,681, where alternating diameters have been used to develop an uneven the surface for the wear side in order to increase the life of a single-layer forming fabric. No certain reason for increased life is given, only a surmise that it may be obtained by water flowing along uneven wear-surface and forming a film that lubricates the wear-surface and thus reduces abrasive wear, or that it may be that the provision of larger weft yarns introduces larger wear-surface areas which prolong the life of the fabric. In this application the central idea seems to consist in transversal yarns larger than usual, unlike in the present invention.
Another application of alternating yarn diameters is presented in U.S. Pat. No. 3,603,354. This patent is about a papermakers' felt that has some larger yarns made of incompressible material. The idea of this invention is to keep the shape of the fabric unchanged as the fabric passes through pressure nips and thus to increase void volume compared to fabrics without incompressible yarns. Also in this invention the advantage is achieved by means of the larger yarns of the fabric and not by means of the smaller yarns as in the present invention.
A two-layer wire with an open wear side and a dense paper side is presented in U.S. Pat. No. 4,985,084. In this case the open structure is achieved by leaving every second transversal bottom yarn out and adjusting the weave structure in such a way that every longitudinal yarn weaves with both paper and wear side yarns. This solution makes it possible to provide a papermaking fabric with all necessary properties, such as non-marking surface, good drainage, retention, runability and wear-resistance. The present invention remarkably improves the wear-resistance compared to this wire because the floats of the wear side transverse yarns can be made longer. Also the present invention reduces sleaziness compared to this wire, because the wear side transverse yarns of a smaller diameter stabilize the weave structure.
The prior art wires with an open wear side usually have a sleazy structure either because of missing wear side knuckles of longitudinal yarns or because of extra paper side transversal yarns that require space on the paper side and thus make it necessary to decrease the density of the transverse yarns that form the basic structure of the wire. An example of extra transverse yarns on the paper side is presented in U.S. Pat. No. 4,423,755. These so-called extra wefts do not strengthen the structure of the wire, because they do not interweave with the yarns of the basic structure, but only pass under the floats of the longitudinal yarns between two transverse yarns of the basic structure. These extra wefts lie quite loose on the paper side of the wire and practically no tension is directed to them.